Fig. 4. The influence, in stingrays acclimated to ion-poor Rio Negro water, of acute exposure to pH 4.0 for 2 h, followed by recovery at control pH 6.1 for 2 h, on (A) unidirectional (J_in, J_out) and net flux rates of Na⁺ (open bars) and Cl^- (hatched bars) and (B) net ammonia flux rates (J_amm; filled bars). Values are means ± 1 S.E.M. (N=5). Standard errors have been omitted from net Na⁺ and Cl^- net flux rates (shaded bars) for clarity as they sometimes overlap those of the unidirectional flux rates. Positive (upward) values represent uptake into the fish; negative (downward) bars represent losses from the fish. Asterisks indicate means significantly different (^*P0.05) from the corresponding control mean.